The present invention relates to a method of manufacturing a GaAs compound semiconductor device.
A conventional GaAs compound semiconductor device such as GaAs MESFET has a well known self-aligned structure formed by using a metal of high melting point as a gate electrode material. In this structure after making the gate electrode source and drain layers of required n+ high impurity concentration are formed by an ion implantation and then an annealing process is performed at annealing temperature of 800.degree. to 900.degree. C. in order to make the source and drain layers activated.
While in order to obtain an ohmic contact to an n+ GaAs region of a semiconductor body, alloys of AuGe/Au or AuGe/Ni/Au for electrode are utilized. In this case 400.degree.-450.degree. C. is adopted as an alloying temperature.
In case of forming the ohmic contact, however, during alloying process thereof, for example the interface of GaAs and AuGe becomes an uneven surface and deterioration in surface morphology of the ohmic electrode results in an occurrence of a ball-up phenomenon.
In case of forming a connection on the semiconductor body, also, a multi-layer structure of Ti/Pt/Au or Ti/Au is utilized as a wiring material. In this case with the heating process of above 400.degree. C. Au of the AuGe, alloy is reacted with Ti of the connection material. When three layer structure of AuGe/Ni/Au is utilized as the ohmic electrode material in order to prevent the ball-up phenomenon, also, Au of upper most layer of the three layer structure is reacted with Ti of the connection material. The morphology in the surface of connection material and the interface of the ohmic electrode material and the connection material becomes deteriorated so that contact resistance becomes large. When the FET is formed, therefore, after applying the ohmic contact it is not preferable to perform a heating process at an ohmic processing temperature of above 400.degree. C.
In the conventional method of manufacturing the semiconductor device utilizing such an ohmic electrode material an order of forming the ohmic electrode is specified so that when an LSI circuit is realized by utilizing for example GaAsMESFET as one element the degree of freedom of manufacturing steps is substantially limited.